In a motor vehicle of this type, a gear ratio is engaged by connecting the free gear of the respective ratio to the shaft on which that gear is supported. To allow the engagement to take place, the difference in rpm-rates of the rotating parts to be engaged to each other should be at least near zero. The process of establishing the at least approximate equality between the rpm-rates is called synchronization.
There is for example a type of transmission where the synchronization devices connected to each of the free gears will at least approximately equalize the rpm-rates of the parts that are to be coupled to each other during a gear shift and will not allow the complete engagement of the respective gear clutch until the rpm-rates have become at least approximately equal. In the initial phase of a gear shift, the parts to be coupled are first put into contact with each other through a friction surface, so that a frictional drag occurs between the parts dependent on the engagement force and the friction coefficient. The synchronizing device can further include a blocking device that prevents a further engagement as long as the frictional drag torque is present. At the point where the rpm-rates are almost equalized, so that that there is at least close to no frictional traction transmitted between the parts, the blocking device releases its hold and the gear-shifting process can be finished by moving the two parts into form-locking engagement with each other. The position where the further engagement is blocked by the blocking device as long as the rpm-rates are not sufficiently synchronized is referred to as the synchronizing position. There is also another type of transmissions that work without synchronization devices of the foregoing description. The function of delaying the form-locking engagement until approximate equality of the rpm-rates has been attained is performed in another way, for example through a suitable control of the engine or through brakes on the rotary shafts. The rpm-difference is in this case determined by appropriate sensors.
The term “automated shift transmission” as used herein refers to transmissions in which a synchronized shift process according to the foregoing description can be performed automatically. The automated actuation of the gear shift is initiated automatically by the control device based on a number of input quantities. The control of the synchronization in particular represents a very complex and challenging problem. For example, at the synchronization threshold, i.e., at the point of a synchronized gear engagement where the aforementioned blocking device stops the sliding sleeve of the shift clutch from advancing further, there should be a predetermined amount of actuating force acting on the sleeve. This particular amount of force is also referred to as the synchronization force. To enable the control device to set this specific amount of force, the position of the synchronization threshold has to be determined with sufficient accuracy, and/or there has to be a way of setting the synchronization force that is independent of an exactly defined position of the synchronization threshold. To complete the gear shifts in a short time interval in an optimized shift process, it is further of critical importance how the synchronization force is built up and set to the predetermined magnitude, in a manner that takes the relevant parameters into account with a sufficient level of accuracy.
A gear-shifting concept involving the use of elasticity is known per se, for example from the European patent EP 579 532 B1. The elasticity is in this case provided by an auxiliary mechanism for the engagement of the gears in a transmission that is shifted by means of cables or rod linkages, where the shift sequence is transmitted through a shift-controller shaft whose rotary movements are driven by a cable or rod. A mechanical connection between the cable or rod and the shaft consists of two pivoted parts whose movements relative to each other are elastically controlled by a spring that stores energy by being compressed during the synchronization phase and subsequently releases the stored energy. The auxiliary mechanism is intended to remove certain drawbacks of conventional gear-shifting devices, specifically the duration of the synchronization time interval that is perceived as too long, or the long free travel of the shift movement, the considerable force required to transmit the shifting action which manifests itself through slowness of the synchronization, and the tactile feeling of shifting noise from the meshing of the gear teeth which is felt as annoying. The auxiliary mechanism described in this reference relates to manually shifted transmissions.
Another reference, EP 695 892 B1, relates to a transmission-shifting system with at least one actuator device as well as sliding sleeves actuated by the device, where the connecting mechanism includes a spring. The transmission as disclosed prevents excessive current levels in the electric motor of the actuator in cases where a gear shift cannot take place immediately. The transmission-shifting system disclosed is an automated shift transmission in the sense of the foregoing definition.
The elastic shift arrangement disclosed in EP 579 532 B1 performs the function of an energy collector by being compressed at certain times and subsequently expanding again. In this manner, the sliding sleeve receives a stronger impulse from the spring than the impulse that could be transmitted to the clutch if the driver performed a fast shift lever movement. The resultant impulse is always dependent on the speed or force that the driver applies to the shift lever, while the precise setting of the required target force at the sliding sleeve is more or less left to chance.
The elastic shifting concept disclosed in EP 695 892 B1 likewise follows the concept of storing energy during a shift process through compression and subsequently releasing the stored energy. The resulting shift elasticity in this case allows the shifter sleeve to follow the actuator movement with a time delay to allow for synchronization to take place. The elastic shift behavior protects the electric motor against overload, but it is not possible to influence the target force level on the sliding sleeve through the speed of the shift lever movement, taking the shift elasticity into account.
The system between the sliding sleeve and the actuator drive source with the connecting mechanism and its kinematic and elastic properties has a high degree of complexity, particularly in the case of automated shift transmissions. So far, no satisfactory solution has been found for performing a synchronized gear shift that meets the multitude of requirements, particularly on how to control the speed of gear engagement while taking into account the elastic characteristics of the mechanism between the actuator drive source and the sliding sleeve.